Ondansetron as primary antiemetic?

Forum Lieutenant

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Every protocol I’ve ever seen indicates Dimenhydrinate as the primary antiemetic and specifically indicated for nausea secondary to narcotic administration. I’m also not aware of Ondansetron’s efficacy for narcotics. Despite this, almost every patient I’ve picked-up for transfer has received Ondansetron insead of Dimenhydrinate for profalaxis of narcotic induced nausea. Moreover, it seems to be very popular in the USA.

So what’s the deal? Why is Ondansetron so pervasive intrahospital and in the USA? Dimenhydrinate is obviously known to cause drowsiness. Are people simply trying to avoid this secondary effect?

Forum Captain

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I've heard rumblings of diphenhydramine possibly correlating with increased rates of dementia for long term users and is now being recommended for short term use only. As for your specific question, I don't know... but people generally very much dislike the feeling of lethargy brought on by diphenhydramine and its derivatives, so it wouldn't surprise me if Zofran has become the de-facto anti-emetic due to this.

Forum Troll

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In my area in the USA, I have never heard of nor seen Dimenhydrinate used in any form for prehospital or in hospital treatment of nausea/vomiting. The first line antiemetic is Ondansetron with a secondary option of Promethazine.

My experience with Ondansetron for nausea post narcotic administration has actually been very favorable when we were using morphine. We switched several years ago in favor of Fentanyl over Morphine. I have only had a couple of patients have nausea post administration.

As for general patients with nausea, my experience is about 50/50. Either it works great for some people or it doesn’t work at all. Ondansetron was the only way I made it through being in the back of the ambulance the entire time during my medic internship. It’s also starting to be my friend while I adjust to the HEMS environment.

I hear and I forget. I see and I remember. I do and I understand.

NVRob;388322 said:

You forget that all the activities you do hurt when you crash and I am the candyman.

Forum Deputy Chief

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The two medications you mentioned are both antiemetics, but they work in very different ways.

Ondansetron is a 5-HT3 serotonin antagonist.
Diphenhydranate is a 1st generation H1 antagonist.

Diphenhydranate is most often used for motion sickness because histamine is the neurotransmitter used by your inner ear to communicate with the vomiting center in the postrema. It also has some anti-muscarinic properties which is my guess as to why it is also effective for opiate induced nausea. Also, as a 1st generation antihistamine, it crosses the blood-brain-barrier, which is why it causes nausea.

Ondansetron has pretty much been the gold standard of antiemetics for many people. It is a sub class of serotonin antagonists, which work on many areas including the stomach, vagus nerve transmission, and the postrema to inhibit signals reaching the vomiting center.

Picking an antiemetic is all about picking the one to break the correct chain leading to the vomiting center. Motion sickness is caused by histamine from the vestibular apparatus, which is why ondansetron is ineffective for treating motion sickness.

Forum Asst. Chief

The two medications you mentioned are both antiemetics, but they work in very different ways.

Ondansetron is a 5-HT3 serotonin antagonist.
Diphenhydranate is a 1st generation H1 antagonist.

Diphenhydranate is most often used for motion sickness because histamine is the neurotransmitter used by your inner ear to communicate with the vomiting center in the postrema. It also has some anti-muscarinic properties which is my guess as to why it is also effective for opiate induced nausea. Also, as a 1st generation antihistamine, it crosses the blood-brain-barrier, which is why it causes nausea.

Ondansetron has pretty much been the gold standard of antiemetics for many people. It is a sub class of serotonin antagonists, which work on many areas including the stomach, vagus nerve transmission, and the postrema to inhibit signals reaching the vomiting center.

Picking an antiemetic is all about picking the one to break the correct chain leading to the vomiting center. Motion sickness is caused by histamine from the vestibular apparatus, which is why ondansetron is ineffective for treating motion sickness.

Forum Deputy Chief

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Ondansetron is a very "clean" drug with no common side effects. It works quickly and it works well for prevention or treatment of most cases of nausea. Now that it's off-patent, it's not that expensive.

The antihistamines also work well, but hit lots of receptors and thus have lots of side effects. They are much cheaper than ondansetron, which IMO is their only advantage, though the mild sedating effect can sometimes be useful.

The only time I use an antihistamine as an antiemetic is if the nausea appears to be motion-induced, or if the person has severe nausea and I'm basically throwing everything at them that I can in an effort to prevent or treat it.

ED/Prehospital Registered Nurse

Zofran is very well tolerated, has a large therapeutic index, is very effective for a large number of patients, and has a low risk profile for most patients. It also has relatively few side effects or adverse reactions making it preferred by most patients over medications that cause drowsiness or other alteration of sensorium.

I've never given dramamine as a first line anti-emetic for anything other than motion sickness, which is/was only when I was out on a boat or doing fixed wing. Even for ambulance induced motion sickness we just gave zofran for the rapid onset.

In fact, zofran is probably at least 95% of the antiemetics I give. I've given some of the more uncommon drugs for nausea like ativan, propofol, haldol, and even kytril far more than dramamine.

Forum Lieutenant

The two medications you mentioned are both antiemetics, but they work in very different ways.

Ondansetron is a 5-HT3 serotonin antagonist.
Diphenhydranate is a 1st generation H1 antagonist.

Diphenhydranate is most often used for motion sickness because histamine is the neurotransmitter used by your inner ear to communicate with the vomiting center in the postrema. It also has some anti-muscarinic properties which is my guess as to why it is also effective for opiate induced nausea. Also, as a 1st generation antihistamine, it crosses the blood-brain-barrier, which is why it causes nausea.

Ondansetron has pretty much been the gold standard of antiemetics for many people. It is a sub class of serotonin antagonists, which work on many areas including the stomach, vagus nerve transmission, and the postrema to inhibit signals reaching the vomiting center.

Picking an antiemetic is all about picking the one to break the correct chain leading to the vomiting center. Motion sickness is caused by histamine from the vestibular apparatus, which is why ondansetron is ineffective for treating motion sickness.

So all this I knew, but both act centrally on the RAS which is where the “vomiting centre” is located. This is why both are capable of controlling drug-induced nausea and vomiting. However, the vast majority of Canadian provinces only use Dimenhydrinate. *see below Perhaps it’s the primary agent becuase it’s effective against vertigo as well as narcotic-induces N&V?

Also - I feel it should be mentioned that Ondansetron can cause Long QT Syndrome and should be used cautiously for anyone with additional risk factors or pathologies.

British Columbia, Alberta, Saskatchewan, Ontario, Nova Scotia and probably more Canadian provinces all use Dimenhydrinate as the primary -and with exception of Ontario flight medics and Alberta- ONLY antiemetic.

Forum Deputy Chief

So all this I knew, but both act centrally on the RAS which is where the “vomiting centre” is located. This is why both are capable of controlling drug-induced nausea and vomiting. However, the vast majority of Canadian provinces only use Dimenhydrinate. *see below Perhaps it’s the primary agent becuase it’s effective against vertigo as well as narcotic-induces N&V?

Also - I feel it should be mentioned that Ondansetron can cause Long QT Syndrome and should be used cautiously for anyone with additional risk factors or pathology.

The vomiting center is basically just a message recipient center. Diphenhydranate blocks histamine from reaching it, and Ondansetron blocks serotonin. Both neurotransmitters come from different sources. They don't just "act" on the vomiting center to stop vomiting. ie giving Zofran and blocking serotonin when the nausea isn't being caused by serotonin won't do anything.

Diphenhydranate is also an anti-muscarinic, and mACh is used for gut stimulation as well as is a receptor on the vomiting center. It's my guess that this specifically is why diphenhydranate is effective for opiates. Simply blocking the histamine receptors would not stop nausea caused by opiates.

EMS Edumacator

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The study that noted prolonged QT was observed following large 32 mg doses of Ondansetron, not the typical 4mg or 8mg dose we use in the field.

A follow up was study was done and while researchers did note instances of prolonged QT, this line from the study is the keeper, “...given that ondansetron is the most widely used drug in the emergency department, if there was a clinically significant risk of cardiac events, it would be plausible to suggest that such events would have limited the drug’s use by now.”

Flight Nurse

The study that noted prolonged QT was observed following large 32 mg doses of Ondansetron, not the typical 4mg or 8mg dose we use in the field.

A follow up was study was done and while researchers did note instances of prolonged QT, this line from the study is the keeper, “...given that ondansetron is the most widely used drug in the emergency department, if there was a clinically significant risk of cardiac events, it would be plausible to suggest that such events would have limited the drug’s use by now.”

Ya outside of a few circumstances such as oncology units where massive doses are common, pre-existing Long QT syndromes, or in conjunction with other QT prolonging drugs the risk of actual arrhythmias is probably very low. But always good to keep in mind.

Chase

*My statements are my opinion and do not reflect those of my employer*

ED/Prehospital Registered Nurse

The study that noted prolonged QT was observed following large 32 mg doses of Ondansetron, not the typical 4mg or 8mg dose we use in the field.

A follow up was study was done and while researchers did note instances of prolonged QT, this line from the study is the keeper, “...given that ondansetron is the most widely used drug in the emergency department, if there was a clinically significant risk of cardiac events, it would be plausible to suggest that such events would have limited the drug’s use by now.”

I've seen it a couple of times, but only under very uncommon circumstance. I've seen in in a couple of psych patients who were on other meds, but that can't be blamed just on the zofran. I've also seen in from parents who give adult dosing that they had at home to their small infants or by OSHEDs, but when you look at the weight based dosing we are back to the levels that they used to be giving to oncology patients. I've never had to worry about it outside of pysch or cards patients, but typically we just print a rhythm strip and call it good.